In a radio cellular system typified by a portable telephone, and the like, service patterns recently become diversified, transmission of large-volume data, such as stationary pictures and moving pictures, as well as transmission of audio data, are requested. 3.5G (HSDPA) terminals have come on a market for a portable telephone, and communication of download traffic speed 14 Mbps is possible.
Hybrid ARQ (hereinafter abbreviated as H-ARQ) is a technique newly introduced from 3.5G. This is a technique pertaining to retransmission control employed when information transmission has ended in a failure. A probability of successful reception achieved during retransmission is enhanced by use of the technique along with use of information about signals received in the past stored in a receiving end, and an improvement in throughput can be expected.
Cellular system standardization specifications of 3.5G (3GPP Long Term Evolution: hereinafter abbreviated as LTE) or a subsequent technique require a peak rate of 100 Mbps for download traffic, and introduction of a MIMO (Multi-Input Multi-Output) system utilizing a plurality of antennas is discussed as a technique for fulfilling the requirement. Use of single user MIMO (SU-MIMO) and multiuser MIMO (MU-MIMO) are discussed as a spatial multiplexing technique for use in the MIMO system.
Single user MIMO limits allocation of a spatial resource to a single user, and enhancement of a peak rate can be expected. In single user MIMO, user multiplexing is carried out in connection with a time axis or a frequency axis; hence, user interference does not occur in connection with a spatial axis. In the meantime, in connection with multiuser MIMO, the spatial resource is allocated to a plurality of users, whereby a multi-user diversity gain is acquired. Hence, enhancement of a sector (cell) throughput can be expected. In particular, when the radius of the cell is large, a throughput is greatly enhanced.
H-ARQ and multiuser MIMO are techniques that enable enhancement of a throughput in a harsh receiving environment, such as that achieved at a position close to the edge of a cell. The fact is that a retransmission control method in the multiuser MIMO system is not sufficiently discussed, and a high throughput is not always gained at all times.
FIGS. 35A and 35B show, as a related-art technique, the outline of a retransmission control method for use in the currently-discussed multiuser MIMO system. A related art of this type is disclosed in; for instance, Non-Patent Document 1. In FIG. 35, a multiuser MIMO system is assumed to have two antennas for a radio base station (BS: Base Station), two user equipments (UE: User Equipment); and two antennas for the respective user equipments (UE1, UE2). FIG. 35A shows first transmission, and FIG. 35B shows second transmission.
In the multiuser MIMO system, user equipments UE1, UE2 feed back one CQI (Channel Quality Indicator) to the radio base station. The radio base station BS assigns the user equipments (UE1, UE2) to the respective antennas in accordance with a feedback CQI, and a data stream is transmitted to the respective user equipments. The respective user equipments UE1, UE2 detect an error in the received data stream. When no error is found in control information, an ACK (Acknowledgement) signal is fed back to the radio base station BS. When an error is found, an NACK (Not Acknowledgement) signal is fed back to the same.
At this time, the user equipment which has fed back the NACK signal buffers (stores) the received signal in preparation for retransmission. The respective user equipment UE1, UE2 measure a receiving SINR (Signal-to-Interference plus Noise power Ratio: a signal-to-interference noise power ratio), and feed back the thus-measured SINR as a CQI to the radio base station BS. In the next transmission operation, when received an ACK signal in accordance with the control information (ACK/NACK) received from the respective user equipments, the radio base station transmits a new data stream. When received a NACK signal, the radio base station BS transmits a retransmission data stream. The user equipment to which retransmission data have been transmitted combines the past-received signal stored in the buffer with retransmission data by means of soft combining, whereby a probability of successful receipt is enhanced. Retransmission control is independently performed for each user equipment.
In the example shown in FIGS. 35A and 35B, the user equipment UE1 is assumed to feed back an ACK signal, and the user equipment UE2 is assumed to feed back a NACK signal. Therefore, new data are transmitted to the user equipment UE1 in the next transmission period, and retransmission data are transmitted to the user equipment UE2 in the next transmission period.    Non-Patent Document 1: 3GPP TSG RAN WG1 Meeting #45, R1-061323, Samsung, “Dynamic Switching between Single and Multi-User MIMO,” 2006